X-ray imaging technology has been employed in a wide range of applications from medical imaging to detection of unauthorized objects or materials in baggage, cargo or other containers generally opaque to the human eye. X-ray imaging typically includes passing high energy radiation (i.e., X-rays) through an object to be imaged. X-rays from a source passing through the object interact with the internal structures of the object and are altered according to various characteristics of the material (e.g., transmission, scattering and diffraction characteristics, etc.) that the X-rays encounter. By measuring changes (e.g., attenuation, energy spectrum, scatter angle, etc.) in the X-ray radiation that exits the object, information related to characteristics of the material such as density, atomic structure and/or atomic number, etc., may be obtained.
Many X-ray scanning systems employ electron beam (e-beam) technology to generate X-rays that penetrate an object of interest to investigate the object's properties. In e-beam technology, an e-beam is directed to impinge on the surface of a target responsive to the e-beam. The target may be formed from, for example, tungsten, molybdenum, gold, or other material that emits X-rays in response to an e-beam impinging on its surface. For example, the target may be a material that converts energy in the e-beam into relatively high energy photons, emitted from the target essentially in the 4π directions. The released energy may be shaped or collimated by blocking selected portions of the X-rays emitted from the target using any of various radiation absorbing material (such as lead). For example, the X-ray may be collimated to form a cone beam, a fan beam, a pencil beam or any other X-ray beam having generally desired characteristics. The collimated X-rays may then pass into an inspection region to penetrate an object of interest to ascertain one or more characteristics of the object.
An e-beam may be generated, for example, from an electron source, the electrons being accelerated and directed as desired along the surface of the target. For example, a generated e-beam may be directed magnetically by bending the beam using one or more magnetic coils, herein referred to as steering coils. In general, the e-beam propagates in a vacuum chamber until the e-beam impinges on the target. Various methods (e.g., bending an e-beam using one or more magnets) of steering an e-beam along a desired path over a surface of the target are well known in the art.
To measure X-ray radiation penetrating an object to be imaged, an array of detectors responsive to X-ray radiation typically is arranged about the object being imaged. Each detector in the array responds to X-rays impinging on its surface to provide a radiograph or view indicative of the total absorption (i.e., attenuation) incurred by material substantially in a line between the X-ray source and a detector in the array. The term “X-ray source” refers generally to an origin or origins of X-ray radiation. In e-beam technology, the X-ray source is typically the locations or points at which the e-beam impinges on the target, thus emitting X-rays from those locations in response to the e-beam. The X-ray source and detector array may be moved relative to one another to obtain a number of views of the object at different angles.
Conventional X-ray systems that are used for computed tomography (CT) scanning establish a circular relationship between an X-ray source and its corresponding detector array. For example, an X-ray source and corresponding detector array may both be rotated together along a circular path. Alternatively, a stationary circular array of detectors may be provided and a source may be rotated about a portion of a circular path (e.g., by providing an e-beam along a circular scanning path on a target). X-ray systems having a circular geometry typically arrange detectors (or detector locations) equidistant from a common point. To generate X-rays that penetrate an inspection region over a number of different viewing angles (e.g., over 180°), a circular target arranged substantially concentric with and diametrically opposed to the detectors is often employed. An e-beam is then typically directed generally in a line through the center point and then deflected such that the e-beam impinges on the target along a circular path. The resulting X-rays then penetrate the object of interest at a desired number of angles or views.